“Modification of a Hybrid Sol-Gel Dielectric and BaTiO3 for Capacitors with Ultrahigh Energy Density”

Abstract:

Demand for energy storage systems that are more efficient and capable of storing greater amounts of energy is increasing in parallel with the growing global need to both save energy and improve energy conversion. Batteries, capacitors and fuel cells have been essential to this effort to develop advanced energy storage devices. Traditionally, owing to their high power density, capacitors have been widely used for decades in various areas such as electronics, defense and medical fields. In addition, capacitors have additional advantages including the capability of fast charging and discharging and long cycle life. Despite their clear strengths, the low energy density of capacitors has limited their more widespread use.

Three approaches have been proposed in an effort to improve the electrical properties of capacitors including energy density and breakdown strength. First, the incorporation of octadecylphosphonic acid (ODPA) monolayers on top of 2-cyanoethyltrimethoxysilane (CNETMS) sol-gel films was performed using different dip coating and post heat treatment conditions. Dip coating under vacuum enabled the formation of ODPA monolayers, at least partially, on CNETMS, which helped to improve maximum discharge energy densities of the capacitor devices. Additional heat treatment further enhanced breakdown strength and device reliability. Despite the improved electrical performance, the degree of alignment or uniformity of the ODPA monolayer could not be clearly confirmed using only FT-IR and AFM so additional characterization techniques should be followed.

Barium titanate (BT) is a dielectric material with a large permittivity yet low breakdown strength. BT thin films were deposited by RF sputtering to construct a multilayer capacitor with Si3N4. Changing the process atmosphere from argon to oxygen and elevating the sputtering temperature to 400 °C did not facilitate the formation of the tetragonal BT phase. However, the use of post-deposition annealing enabled the conversion from amorphous to tetragonal. Temperatures as high as 1100 °C will be investigated to further increase the degree of tetragonal phase before fabrication of capacitors with Si3N4.

BT also can be used for capacitors in the form of nanoparticles when incorporated in a polymer matrix with a high breakdown strength. However, the large gap in permittivity between the BT nanoparticles and the polymeric host causes electric field concentration to form that result in the breakdown of the overall dielectric. In order to mitigate this problem, a buffer layer of ZrO2 has been considered. The fabrication of uniform ZrO2 shells on the surface of the BT nanoparticles has not yet been optimized but has since been verified to successfully produce ZrO2 from its precursor ZrOCl2-8H2O.